Physiological sensor having reduced sensitivity to interference

ABSTRACT

A physiological sensor having reduced sensitivity to interference includes a light source, a light detector in optical communication with the light source, and a sensor pad at least partially housing the light source and the light detector. The sensor pad is configured to be capacitively isolated from a patient. Moreover, the physiological sensor may be electrically connected to an amplifier having a signal ground and a monitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/471,716 filed on May 26, 2009, the contents of which are incorporatedby reference herein in their entirety.

BACKGROUND

Physiological sensors are often used in medical applications to helpdoctors diagnose, monitor, and treat patients. Some physiologicalsensors use spectroscopy to provide valuable information about thepatient's body tissue. Spectroscopy generally refers to the dispersionof light as it travels through a medium. A physiological sensoremploying near-infrared spectroscopy may be used to detectcharacteristics of various body tissues by transmitting and receivingnear-infrared light through the body tissue, and outputting a signal toa controller that provides valuable information about the body tissue. Adoctor may use this information to diagnose, monitor, and treat thepatient.

To measure the intensity of the light that travels inside the tissue,the near-infrared spectroscopy sensor may use one or more large areaphotodiodes mounted onto a flexible circuit board within a sensor pad.Because the photodiodes have a high equivalent resistance of the p-njunction, the sensor is very sensitive to the electromagneticinterference from other devices, such as electrosurgical equipment,electrocardiogram devices, or power supplies from medical or otherelectronic devices. One way to reduce the sensitivity of thenear-infrared spectroscopy sensor to these other devices includesenclosing the photodiodes in a Faraday shield made from a copper mesh orplastic film covered by a transparent conductive material, such asiridium oxide. However, the Faraday shield is expensive, decreases thesensitivity of the photodiodes to the near-infrared light generated bythe sensor, and reduces the flexibility of the sensor.

Accordingly, a sensor is needed that reduces or eliminates the effectsof electronic devices without the added expense and/or decreasedsensitivity in sensors employing the Faraday shield solution.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an assembly view of an exemplary physiological sensorconfigured to be capacitively isolated from a patient; and

FIG. 2 is an exemplary circuit diagram illustrating an equivalentcircuit of the physiological sensor capacitively isolated from thepatient.

DETAILED DESCRIPTION

A physiological sensor includes a light source, a light detector, and asensor pad that is capacitively isolated from a patient. When the sensorpad is placed on the patient, light from the light source travelsthrough a portion of the patient's body and is at least partiallyreceived by the light detector. The light detector then outputs a signalto a signal ground that is indicative of oxygen saturation. However,electronic devices such as electrosurgical generators, electrocardiogramdevices, power sources, or any other medical or non-medical devices nearthe sensor pad may interfere with the light received by the lightdetector. In particular, the electronic device may create a voltagepotential between the patient and the sensor pad that generates anelectromagnetic field that may be detected by the light detector. Ifdetected, the electromagnetic field may affect the signal output by thelight detector, causing false oxygen saturation readings. To remedythis, the sensitivity of the sensor may be reduced by capacitivelyisolating the sensor pad from the patient, which effectively reduces thevoltage potential between the patient and the sensor pad and reduces thesensitivity of the sensor. Moreover, the sensitivity of the sensor maybe further reduced by capacitively isolating the signal ground from themonitor.

FIG. 1 is an assembly view of an exemplary physiological sensor 10 thatis capacitively isolated from a patient on which the sensor 10 isplaced. The sensor 10 includes a light source 12, such as a lightemitting diode, that may be configured to generate light in anear-infrared region of the electromagnetic spectrum. A light detector14, such as a photodiode, is in optical communication with the lightsource 12, and thus, may be configured to receive light in the samenear-infrared region of the electromagnetic spectrum. This way, thesensor 10 may be configured be a pulse oximeter, tissue oximeter, orother device configured to detect oxygen saturation. Both the lightsource 12 and light detector 14 may be disposed on a sensor pad 16 suchthat the sensor pad 16 at least partially houses the light source 12 andthe light detector 14. In one exemplary approach, the light source 12may be disposed on a different sensor pad 16 than the light detector 14.Moreover, the sensor pad 16 may include any number of light sources 12and/or light detectors 14. The sensor pad 16 includes openings 18 thatallow light generated by the light source 12 to propagate through bodytissue, as well as openings 18 that allow the light detector 14 toreceive the light from the light source 14. To reduce the sensitivity ofthe sensor 10, the sensor pad 16 is capacitively isolated from thepatient to a voltage potential between the patient and the sensor pad16.

In one exemplary approach, a first surface 20 of the sensor pad 16 is atleast partially coated with a conductive adhesive and a second surface22 of the sensor pad 16 is at least partially coated with a pressuresensitive adhesive 26 that is not conductive to capacitively isolate thepatient from the sensor pad 16. The second surface 22 of the sensor pad16 is on the underside of the sensor pad 16 as illustrated in FIG. 1.Although coated on the second surface 22, the pressure sensitiveadhesive 26 is illustrated in FIG. 1 as separate piece than the sensorpad 16 because it would otherwise not be viewable in FIG. 1. Moreover,the openings 18 for the light source 12 and the light detector 14 mayfurther be defined by the second surface 22 and are thus illustrated inFIG. 1 as being further defined by the pressure sensitive adhesive 26.

It is appreciated that one or both of the first and second surfaces 20and 22 may be completely coated with the conductive adhesive and thepressure sensitive adhesive 26, respectively. The conductive adhesivemay be any conductive adhesive, such as ARCare-8001 manufactured byAdhesive Research Corporation. The pressure sensitive adhesive 26 may beany adhesive that will adhere to the patient's skin. When attached, thesensor pad 16 is arranged such that the conductive adhesive is spacedfrom the patient. The distance between the conductive adhesive and thepatient's skin may affect the capacitance C_(p) of the patient,discussed in further detail below (see FIG. 2). For example, thecapacitance C_(p) may be inversely related to the distance between thepatient's skin and the conductive adhesive. In other words, as thedistance between the conductive adhesive and the patient's skinincreases, capacitance C_(p) decreases, and vice versa. Therefore, thethickness of the sensor pad 16 may be designed to make the capacitanceC_(p) sufficiently small to reduce a field produced by the voltagedifference between the patient and the sensor 10, yet large enough tocapacitively isolate the patient from the sensor pad 16.

The capacitance C_(p) may be similar to the capacitance of aparallel-plate capacitor the patient's body represents one plate and theconductive adhesive on the first surface 20 represents the other plate.The first surface 20 has an area A and is separated from the patient'sbody by a distance d. From this, capacitance C_(p) is approximatelyequal to the following:

C _(p)=ε₀ε_(r) A/d   (Equation 1)

In Equation 1, C_(p) is the capacitance in Farads, and as discussedabove, A is the area of overlap of the first surface 20 and thepatient's body measured in square meters, and d is the distance betweenthe first surface 20 and the patient's body measured in meters. Thevalue ε_(r) is the dielectric constant of the material between theplates, which may be approximately equal to 1. The value ε_(O) is thepermittivity of free space where ε₀=8.854×10⁻¹² F/m. Using thisequation, a 3 cm×1 cm sensor pad with a 1 mm gap between the patient'sbody and the first surface 20 would have a capacitance C_(p) ofapproximately 3 pF. However, this value of C_(p) is merely exemplary.

The sensor 10 itself may generate an electromagnetic field that may bereceived by and affect the light detectors 14. For example, the lightsource 12 and light detector 14 may be disposed on a printed circuitboard 28 having traces 30. The spacing and configuration of the traces30 may generate an electromagnetic field that interferes with the lightdetectors 14 in the same way a large voltage potential across thepatient 44 and the sensor pad 16 may generate an electromagnetic fieldand affect the light detectors 14. To compensate for this type ofelectromagnetic field, the traces 30 on the printed circuit board 28 maybe printed very close to one another to reduce the magnitude of anyelectromagnetic field generated therebetween. Moreover, the shape of thetraces 30 may be such that there are few, if any, loops created. Forexample, the traces 30 may be configured to travel parallel to oneanother and in straight lines as much as possible, with few, if any,rounded edges. This will minimize a loop that may pick up a highfrequency electromagnetic field from an interference current generatedby an electrosurgical generator to the ground via the patient, asdiscussed in further detail below.

The sensor 10 may include other components, such as a light-blocking pad32 disposed on the sensor pad 16 and over the printed circuit board 28to reduce interference from ambient light, and a spacer 34 disposedbetween the light-blocking pad 32 and the printed circuit board 28. Asillustrated, the spacer 34 may define openings 18 for each light source12 and light detector 14. Further, the sensor 10 may include a shield 42to protect signals transmitted from the light detector 14 to a signalground 50 (see FIG. 2) from interference. When packaged, the pressuresensitive adhesive 26 may be covered with a liner 38, which may furtherdefine openings 36, and a tab 40 to allow removal of the liner 38 andexpose the pressure sensitive adhesive 26 on the second surface 22 priorto placing the sensor 10 on the patient 44.

Referring now to FIG. 2, the physiological sensor 10 previouslydescribed may be used to detect oxygen saturation of a patient 44without interference from an electronic device, such as anelectrosurgical generator. Although illustrated as an electrosurgicalgenerator, the electronic device 46 may be an electrocardiogram device,power supply, or any other medical or non-medical electronic device. Thesensor 10 is in communication with an amplifier 48 having a signalground 50 and a monitor 52. Namely, the light detector 14 outputs acurrent or voltage signal representative of oxygen saturation. Theamplifier 48 processes the signal and transmits the processed signal tothe monitor 52 where the oxygen saturation may be graphically displayedto a user, such as a medical professional. The signal ground 50 may beconnected to the sensor 10 via the conductive adhesive. In one exemplaryapproach, the conductive adhesive covers the entire first surface 20 ofthe sensor pad 16, and connects to the signal ground 50 via an exposedarea of copper on the printed circuit board 28. This way, the signalground 50 and the sensor 10 are at electrically the same potential,while capacitively isolated from the patient 44. Further, the shield 42protects the signal transmitted from the light detector 14 to the signalground 50 from interference.

The capacitance C_(p) between the patient 44 and the sensor pad 16absorbs changes in voltage between the sensor pad 16 and the patient 44causing interference with the sensor 10, and in particular, anelectromagnetic field received by the light detectors 14. The changes involtage potential may be caused by an electronic device 46 used with thepatient 44 as well as various physical characteristics of the patient44, such as the patient's height, weight, etc, may cause these changesin voltage.

To further reduce interference, the signal ground 50 may be capacitivelyisolated from the monitor 52, represented in FIG. 2 as a signalisolation capacitance C. The isolation capacitance C_(i) includes aparasitic capacitance between components of the monitor 52 and amplifier48, a capacitance between an octo-coupler and a capacitance betweenprimary and secondary windings of an isolation transformer providingelectrical power to the amplifier 48 and sensor 10. In one exemplaryapproach, the capacitance C_(p) between the patient 44 and the sensorpad 16 is greater than the signal isolation capacitance C_(i), creatinga voltage divider that decreases the electrical potential between thepatient 44 and the light source 12. For example, the capacitance C_(p)between the signal ground 50 and the monitor 52 may be 0.1 pF to 1.0 pF,and the capacitance C_(p) between the patient 44 and the sensor pad 16is greater than that. The signal isolation capacitance C_(i) may betuned relative to the capacitance C_(p) between the patient 44 and thesensor pad 16 to control the reduction of interference.

The above description is intended to be illustrative and notrestrictive. Many alternative approaches or applications other than theexamples provided would be apparent to those of skill in the art uponreading the above description. The scope of the invention should bedetermined, not with reference to the above description, but shouldinstead be determined with reference to the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isanticipated and intended that future developments will occur in the artsdiscussed herein, and that the disclosed systems and methods will beincorporated into such future examples. In sum, it should be understoodthat the invention is capable of modification and variation and islimited only by the following claims.

The present embodiments have been particularly shown and described,which are merely illustrative of the best modes. It should be understoodby those skilled in the art that various alternatives to the embodimentsdescribed herein may be employed in practicing the claims withoutdeparting from the spirit and scope as defined in the following claims.It is intended that the following claims define the scope of theinvention and that the method and apparatus within the scope of theseclaims and their equivalents be covered thereby. This description shouldbe understood to include all novel and non-obvious combinations ofelements described herein, and claims may be presented in this or alater application to any novel and non-obvious combination of theseelements. Moreover, the foregoing embodiments are illustrative, and nosingle feature or element is essential to all possible combinations thatmay be claimed in this or a later application.

All terms used in the claims are intended to be given their broadestreasonable constructions and their ordinary meanings as understood bythose skilled in the art unless an explicit indication to the contraryis made herein. In particular, use of the singular articles such as “a,”“the,” “said,” etc. should be read to recite one or more of theindicated elements unless a claim recites an explicit limitation to thecontrary.

1-19. (canceled)
 20. A sensor comprising: a sensor pad; a conductivematerial disposed on the sensor pad; a printed circuit board disposed onthe conductive material, the printed circuit board including a lightsource configured to generate light and a light detector configured toreceive light generated by the light source, wherein the sensor pad hasa thickness that capacitively isolates the conductive material from apatient.
 21. The sensor of claim 20, wherein a capacitance of theconductive material relative to the patient is inversely proportional tothe thickness of the sensor pad.
 22. The sensor of claim 21, wherein thecapacitance is defined as: C_(p)=ε₀ε_(r)A/d, wherein C_(p) is thecapacitance in Farads, A is an area of the sensor pad disposed on thepatient, d is a distance between the conductive material and thepatient, ε_(r) is a dielectric constant between the conductive materialand the patient, and ε₀ is a permittivity of free space.
 23. The sensorof claim 22, wherein the dielectric constant is equal to
 1. 24. Thesensor of claim 22, wherein the permittivity of free space ε₀ is equalto 8.854×10⁻¹² F/m.
 25. The sensor of claim 20, wherein the conductivematerial is electrically connected to the printed circuit board.
 26. Thesensor of claim 25, wherein the conductive material and the printedcircuit board are electrically grounded.
 27. The sensor of claim 20,wherein the printed circuit board includes traces configured to minimizean electromagnetic field generated between the traces.
 28. The sensor ofclaim 20, further comprising a light-blocking pad disposed on the sensorpad and configured to at least partially limit ambient light received bythe light detector.
 29. A system comprising: an amplifier configured toreceive signals representing physiological information, wherein theamplifier includes a monitor configured to graphically display thephysiological information represented by the received signals; and asensor having a sensor pad, a conductive material disposed on the sensorpad, and a printed circuit board disposed on the conductive material,the printed circuit board including a light source configured togenerate light and a light detector configured to receive lightgenerated by the light source, wherein the sensor pad has a thicknessthat capacitively isolates the conductive material from a patient. 30.The system of claim 29, wherein a capacitance of the conductive materialrelative to the patient is inversely proportional to the thickness ofthe sensor pad.
 31. The system of claim 30, wherein the capacitance isdefined as: C_(p)=ε₀ε_(r)A/d, wherein C_(p) is the capacitance inFarads, A is an area of the sensor pad disposed on the patient, d is adistance between the conductive material and the patient, ε_(r) is adielectric constant between the conductive material and the patient, andε₀ is a permittivity of free space.
 32. The system of claim 29, whereinthe sensor pad is capacitively isolated from the monitor.
 33. The systemof claim 29, wherein the conductive material is electrically connectedto the printed circuit board.
 34. The system of claim 33, wherein theconductive material and the printed circuit board are electricallygrounded.
 35. The system of claim 29, wherein the printed circuit boardincludes traces configured to minimize an electromagnetic fieldgenerated between the traces.
 36. The system of claim 29, wherein thesensor includes a light-blocking pad disposed on the sensor pad, whereinthe light-blocking pad is configured to at least partially limit ambientlight received by the light detector.
 37. The system of claim 29,further comprising a shield electrically disposed between the sensor andthe amplifier.
 38. The system of claim 29, wherein the amplifierincludes a signal ground electrically connected to the sensor andcapacitively isolated from the monitor.
 39. A sensor comprising: asensor pad; a conductive material disposed on the sensor pad; a printedcircuit board disposed on the conductive material, the printed circuitboard including a light source configured to generate light and a lightdetector configured to receive light generated by the light source,wherein the sensor pad has a thickness that capacitively isolates theconductive material from a patient, and wherein a capacitance of theconductive material relative to the patient is defined as: C_(p)=ε₀ε_(r)A/d, wherein C_(p) is the capacitance in Farads, A is an area of thesensor pad disposed on the patient, d is a distance between theconductive material and the patient, ε_(r) is a dielectric constantbetween the conductive material and the patient, and ε₀ is apermittivity of free space.